The present invention relates to a transparent shrinkable film comprising a base layer containing a mixture of polypropylene and hydrocarbon resin. The invention also relates to a process for manufacturing such a film and to a shrink label comprising the film.
It is known from "Verpackungsrundschau" [Packaging Review] to surround cylindrical or substantially cylindrical packages, such as bottles or cans with overall labels; these labels, which entirely surround the circumferential surface of the packages are also known as sleeves. The films used for this purpose are, for example, made from polyethylene, polybutylene, polystyrene, copolymers of polyethylene and polypropylene, copolymers of ethylene and vinyl acetate or various blends, but in particular from polyvinyl chloride. To achieve the desired shrink properties in the circumferential direction of the sleeves, which are essential for the intended use, the films usually are oriented by biaxial stretching which is performed, for example, using a bubble, stenter or calendar process, whereby particular attention is paid to the orientation in the transverse direction. To accomplish an absolutely crease-free, tight contact between the sleeve and the package, the shrink values measured after a treatment for about 15 minutes at 90.degree. C. in a circulating air cabinet should be about 20 to 40% in the transverse direction (s.sub.t) and about 7 to 8% in the longitudinal direction (s.sub.l).
The sleeves are provided with prints, for example, by reverse side printing and then converted into a tubing by gluing or welding. Due to the controlled transverse shrink, a tight, crease-free contact is created between the tube and the package in the shrink tunnel. The sleeves are applied automatically with the aid of brushes or manually. In addition to the desired shrink the sleeves must possess the following properties: high gloss, clarity, good slip and stability (corresponding to the product of modulus of elasticity and thickness.sup.3) for the automatic application of the sleeve, good printability and good welding/bonding characteristics.
For an economic large-scale manufacture of sleeves the use of heat-sealable films is of great advantage, for less time is required for sealing than for welding or gluing. Less material is required as well.
Among the known films, PVC films are the films which best fulfill the demands set forth above. Their high shrink capacities and their good optical and mechanical properties make them suitable for a great number of applications. The shape or diameter of a package can vary up to 30% or even more within the surface area to which the sleeve is to be applied.
The disadvantages of PVC sleeves re in particular their high price which mainly results from the high density of 1.39 kg/dm.sup.3 (which is about 50% higher than the density of, for example, polypropylene), and their unsatisfactory heat sealability. Furthermore, problems are encountered with regard to corrosion of the manufacturing and processing apparatus (see EP-A-0 233 400).
EP-A-0 051 480 describes a heat-sealable film on a PVC basis. However, the production of this film is very expensive because it comprises five layers built up in the order of sequence ABCBA. The B-layer is an adhesion-promoting layer comprising an ethylene/vinyl acetate copolymer.
It is also known to use blends based on olefinic homo-, co- and terpolymers for producing sleeves, apart from PVC. Japanese Application JP-A-22 4 868 (Toray) discloses, for example a heat-shrinkable multilayer film based on polypropylene and having the layer build-up configuration AB or ABA. Layer A is comprised of a random ethylene/propylene copolymer and layer B is a blend of a random ethylene/propylene copolymer and a propylene/butylene copolymer. The film is said to have a shrink s.sub.t, in the circumferential direction of the sleeve of 15% or more at 100.degree. C., and furthermore is sealable due to the selected layer build-up. However, the mechanical strength of this film is quite poor. For example, PVC has a modulus of elasticity in the longitudinal direction (E.sub.t) of about 2,600 to 2,900 N/mm.sup.2 and in the transverse direction (E.sub.t) of about 3,500 to 3,800 N/mm.sup.2, whereas for the polyolefinic films E.sub.l is only about 100 to 1,100 N/mm.sup.2 and E.sub.t is only about 100 to 1,500 N/mm.sup.2. To achieve mechanical strength values which come up to those of PVC films, the thickness of the polyolefinic film has to be increased by about 50%, which in turn leads to correspondingly higher costs. Further disadvantages of the polyolefinic film according to JP-A-224 868 compared to PVC are its lower gloss and higher haze.
From EP-A-0 171 733 films of the type described above are known which consist of copolymers of propylene with other alpha-olefins and a resin admixture and which possess acceptable shrink values. However, the mechanical properties of the disclosed films are not satisfactory, and they are not sealable.